diff --git a/.coveragerc b/.coveragerc
index 653d88b..766c60d 100644
--- a/.coveragerc
+++ b/.coveragerc
@@ -2,7 +2,6 @@
 branch = True
 
 [report]
-fail_under = 100
 show_missing = True
 omit =
     google/cloud/billing/budgets/__init__.py
diff --git a/.github/.OwlBot.lock.yaml b/.github/.OwlBot.lock.yaml
index 864c176..e2b39f9 100644
--- a/.github/.OwlBot.lock.yaml
+++ b/.github/.OwlBot.lock.yaml
@@ -1,3 +1,3 @@
 docker:
   image: gcr.io/repo-automation-bots/owlbot-python:latest
-  digest: sha256:4c981a6b6f2b8914a448d7b3a01688365be03e3ed26dfee399a6aa77fb112eaa
+  digest: sha256:99d90d097e4a4710cc8658ee0b5b963f4426d0e424819787c3ac1405c9a26719
diff --git a/.kokoro/samples/python3.9/common.cfg b/.kokoro/samples/python3.9/common.cfg
new file mode 100644
index 0000000..10b99b5
--- /dev/null
+++ b/.kokoro/samples/python3.9/common.cfg
@@ -0,0 +1,40 @@
+# Format: //devtools/kokoro/config/proto/build.proto
+
+# Build logs will be here
+action {
+  define_artifacts {
+    regex: "**/*sponge_log.xml"
+  }
+}
+
+# Specify which tests to run
+env_vars: {
+    key: "RUN_TESTS_SESSION"
+    value: "py-3.9"
+}
+
+# Declare build specific Cloud project.
+env_vars: {
+    key: "BUILD_SPECIFIC_GCLOUD_PROJECT"
+    value: "python-docs-samples-tests-py39"
+}
+
+env_vars: {
+    key: "TRAMPOLINE_BUILD_FILE"
+    value: "github/python-billingbudgets/.kokoro/test-samples.sh"
+}
+
+# Configure the docker image for kokoro-trampoline.
+env_vars: {
+    key: "TRAMPOLINE_IMAGE"
+    value: "gcr.io/cloud-devrel-kokoro-resources/python-samples-testing-docker"
+}
+
+# Download secrets for samples
+gfile_resources: "/bigstore/cloud-devrel-kokoro-resources/python-docs-samples"
+
+# Download trampoline resources.
+gfile_resources: "/bigstore/cloud-devrel-kokoro-resources/trampoline"
+
+# Use the trampoline script to run in docker.
+build_file: "python-billingbudgets/.kokoro/trampoline.sh"
\ No newline at end of file
diff --git a/.kokoro/samples/python3.9/continuous.cfg b/.kokoro/samples/python3.9/continuous.cfg
new file mode 100644
index 0000000..a1c8d97
--- /dev/null
+++ b/.kokoro/samples/python3.9/continuous.cfg
@@ -0,0 +1,6 @@
+# Format: //devtools/kokoro/config/proto/build.proto
+
+env_vars: {
+    key: "INSTALL_LIBRARY_FROM_SOURCE"
+    value: "True"
+}
\ No newline at end of file
diff --git a/.kokoro/samples/python3.9/periodic-head.cfg b/.kokoro/samples/python3.9/periodic-head.cfg
new file mode 100644
index 0000000..f9cfcd3
--- /dev/null
+++ b/.kokoro/samples/python3.9/periodic-head.cfg
@@ -0,0 +1,11 @@
+# Format: //devtools/kokoro/config/proto/build.proto
+
+env_vars: {
+    key: "INSTALL_LIBRARY_FROM_SOURCE"
+    value: "True"
+}
+
+env_vars: {
+    key: "TRAMPOLINE_BUILD_FILE"
+    value: "github/python-pubsub/.kokoro/test-samples-against-head.sh"
+}
diff --git a/.kokoro/samples/python3.9/periodic.cfg b/.kokoro/samples/python3.9/periodic.cfg
new file mode 100644
index 0000000..50fec96
--- /dev/null
+++ b/.kokoro/samples/python3.9/periodic.cfg
@@ -0,0 +1,6 @@
+# Format: //devtools/kokoro/config/proto/build.proto
+
+env_vars: {
+    key: "INSTALL_LIBRARY_FROM_SOURCE"
+    value: "False"
+}
\ No newline at end of file
diff --git a/.kokoro/samples/python3.9/presubmit.cfg b/.kokoro/samples/python3.9/presubmit.cfg
new file mode 100644
index 0000000..a1c8d97
--- /dev/null
+++ b/.kokoro/samples/python3.9/presubmit.cfg
@@ -0,0 +1,6 @@
+# Format: //devtools/kokoro/config/proto/build.proto
+
+env_vars: {
+    key: "INSTALL_LIBRARY_FROM_SOURCE"
+    value: "True"
+}
\ No newline at end of file
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 4f00c7c..62eb5a7 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -16,7 +16,7 @@
 # See https://pre-commit.com/hooks.html for more hooks
 repos:
 -   repo: https://github.com/pre-commit/pre-commit-hooks
-    rev: v3.4.0
+    rev: v4.0.1
     hooks:
     -   id: trailing-whitespace
     -   id: end-of-file-fixer
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 07aa656..faead23 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,21 @@
 # Changelog
+## [1.3.0](https://www.github.com/googleapis/python-billingbudgets/compare/v1.2.0...v1.3.0) (2021-06-30)
+
+
+### Features
+
+* add always_use_jwt_access ([#107](https://www.github.com/googleapis/python-billingbudgets/issues/107)) ([af01c04](https://www.github.com/googleapis/python-billingbudgets/commit/af01c04f8c4ea1e052b186733527838fbdd31bd1))
+
+
+### Bug Fixes
+
+* disable always_use_jwt_access ([#112](https://www.github.com/googleapis/python-billingbudgets/issues/112)) ([e9ffec8](https://www.github.com/googleapis/python-billingbudgets/commit/e9ffec8dafcdcc2692050326daa8d3ac13b0e63a))
+
+
+### Documentation
+
+* omit mention of Python 2.7 in 'CONTRIBUTING.rst' ([#1127](https://www.github.com/googleapis/python-billingbudgets/issues/1127)) ([#102](https://www.github.com/googleapis/python-billingbudgets/issues/102)) ([3d88e32](https://www.github.com/googleapis/python-billingbudgets/commit/3d88e32d967e97a72b8162bd0ed19552aa4c253e)), closes [#1126](https://www.github.com/googleapis/python-billingbudgets/issues/1126)
+
 ## [1.2.0](https://www.github.com/googleapis/python-billingbudgets/compare/v1.1.1...v1.2.0) (2021-05-17)
 
 ### Features
diff --git a/CONTRIBUTING.rst b/CONTRIBUTING.rst
index ef8f755..29ebaa3 100644
--- a/CONTRIBUTING.rst
+++ b/CONTRIBUTING.rst
@@ -68,15 +68,12 @@ Using ``nox``
 We use `nox <https://nox.readthedocs.io/en/latest/>`__ to instrument our tests.
 
 - To test your changes, run unit tests with ``nox``::
+    $ nox -s unit
 
-    $ nox -s unit-2.7
-    $ nox -s unit-3.8
-    $ ...
+- To run a single unit test::
 
-- Args to pytest can be passed through the nox command separated by a `--`. For
-  example, to run a single test::
+    $ nox -s unit-3.9 -- -k <name of test>
 
-    $ nox -s unit-3.8 -- -k <name of test>
 
   .. note::
 
@@ -143,8 +140,7 @@ Running System Tests
 - To run system tests, you can execute::
 
    # Run all system tests
-   $ nox -s system-3.8
-   $ nox -s system-2.7
+   $ nox -s system
 
    # Run a single system test
    $ nox -s system-3.8 -- -k <name of test>
@@ -152,9 +148,8 @@ Running System Tests
 
   .. note::
 
-      System tests are only configured to run under Python 2.7 and
-      Python 3.8. For expediency, we do not run them in older versions
-      of Python 3.
+      System tests are only configured to run under Python 3.8.
+      For expediency, we do not run them in older versions of Python 3.
 
   This alone will not run the tests. You'll need to change some local
   auth settings and change some configuration in your project to
@@ -218,8 +213,8 @@ Supported versions can be found in our ``noxfile.py`` `config`_.
 .. _config: https://github.com/googleapis/python-billingbudgets/blob/master/noxfile.py
 
 
-We also explicitly decided to support Python 3 beginning with version
-3.6. Reasons for this include:
+We also explicitly decided to support Python 3 beginning with version 3.6.
+Reasons for this include:
 
 -  Encouraging use of newest versions of Python 3
 -  Taking the lead of `prominent`_ open-source `projects`_
diff --git a/docs/conf.py b/docs/conf.py
index 3f08a1e..7eb54c8 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -80,9 +80,9 @@
 master_doc = "index"
 
 # General information about the project.
-project = u"google-cloud-billing-budgets"
-copyright = u"2019, Google"
-author = u"Google APIs"
+project = "google-cloud-billing-budgets"
+copyright = "2019, Google"
+author = "Google APIs"
 
 # The version info for the project you're documenting, acts as replacement for
 # |version| and |release|, also used in various other places throughout the
@@ -281,7 +281,7 @@
     (
         master_doc,
         "google-cloud-billing-budgets.tex",
-        u"google-cloud-billing-budgets Documentation",
+        "google-cloud-billing-budgets Documentation",
         author,
         "manual",
     )
@@ -316,7 +316,7 @@
     (
         master_doc,
         "google-cloud-billing-budgets",
-        u"google-cloud-billing-budgets Documentation",
+        "google-cloud-billing-budgets Documentation",
         [author],
         1,
     )
@@ -335,7 +335,7 @@
     (
         master_doc,
         "google-cloud-billing-budgets",
-        u"google-cloud-billing-budgets Documentation",
+        "google-cloud-billing-budgets Documentation",
         author,
         "google-cloud-billing-budgets",
         "google-cloud-billing-budgets Library",
@@ -363,6 +363,7 @@
     "google.api_core": ("https://googleapis.dev/python/google-api-core/latest/", None,),
     "grpc": ("https://grpc.github.io/grpc/python/", None),
     "proto-plus": ("https://proto-plus-python.readthedocs.io/en/latest/", None),
+    "protobuf": ("https://googleapis.dev/python/protobuf/latest/", None),
 }
 
 
diff --git a/docs/multiprocessing.rst b/docs/multiprocessing.rst
index 1cb29d4..536d17b 100644
--- a/docs/multiprocessing.rst
+++ b/docs/multiprocessing.rst
@@ -1,7 +1,7 @@
 .. note::
 
-   Because this client uses :mod:`grpcio` library, it is safe to
+   Because this client uses :mod:`grpc` library, it is safe to
    share instances across threads. In multiprocessing scenarios, the best
    practice is to create client instances *after* the invocation of
-   :func:`os.fork` by :class:`multiprocessing.Pool` or
+   :func:`os.fork` by :class:`multiprocessing.pool.Pool` or
    :class:`multiprocessing.Process`.
diff --git a/google/cloud/billing/budgets_v1/services/budget_service/transports/base.py b/google/cloud/billing/budgets_v1/services/budget_service/transports/base.py
index c9f9307..8530308 100644
--- a/google/cloud/billing/budgets_v1/services/budget_service/transports/base.py
+++ b/google/cloud/billing/budgets_v1/services/budget_service/transports/base.py
@@ -24,6 +24,7 @@
 from google.api_core import gapic_v1  # type: ignore
 from google.api_core import retry as retries  # type: ignore
 from google.auth import credentials as ga_credentials  # type: ignore
+from google.oauth2 import service_account  # type: ignore
 
 from google.cloud.billing.budgets_v1.types import budget_model
 from google.cloud.billing.budgets_v1.types import budget_service
@@ -47,8 +48,6 @@
     except pkg_resources.DistributionNotFound:  # pragma: NO COVER
         _GOOGLE_AUTH_VERSION = None
 
-_API_CORE_VERSION = google.api_core.__version__
-
 
 class BudgetServiceTransport(abc.ABC):
     """Abstract transport class for BudgetService."""
@@ -69,6 +68,7 @@ def __init__(
         scopes: Optional[Sequence[str]] = None,
         quota_project_id: Optional[str] = None,
         client_info: gapic_v1.client_info.ClientInfo = DEFAULT_CLIENT_INFO,
+        always_use_jwt_access: Optional[bool] = False,
         **kwargs,
     ) -> None:
         """Instantiate the transport.
@@ -92,6 +92,8 @@ def __init__(
                 API requests. If ``None``, then default info will be used.
                 Generally, you only need to set this if you're developing
                 your own client library.
+            always_use_jwt_access (Optional[bool]): Whether self signed JWT should
+                be used for service account credentials.
         """
         # Save the hostname. Default to port 443 (HTTPS) if none is specified.
         if ":" not in host:
@@ -101,7 +103,7 @@ def __init__(
         scopes_kwargs = self._get_scopes_kwargs(self._host, scopes)
 
         # Save the scopes.
-        self._scopes = scopes or self.AUTH_SCOPES
+        self._scopes = scopes
 
         # If no credentials are provided, then determine the appropriate
         # defaults.
@@ -120,13 +122,20 @@ def __init__(
                 **scopes_kwargs, quota_project_id=quota_project_id
             )
 
+        # If the credentials is service account credentials, then always try to use self signed JWT.
+        if (
+            always_use_jwt_access
+            and isinstance(credentials, service_account.Credentials)
+            and hasattr(service_account.Credentials, "with_always_use_jwt_access")
+        ):
+            credentials = credentials.with_always_use_jwt_access(True)
+
         # Save the credentials.
         self._credentials = credentials
 
-    # TODO(busunkim): These two class methods are in the base transport
+    # TODO(busunkim): This method is in the base transport
     # to avoid duplicating code across the transport classes. These functions
-    # should be deleted once the minimum required versions of google-api-core
-    # and google-auth are increased.
+    # should be deleted once the minimum required versions of google-auth is increased.
 
     # TODO: Remove this function once google-auth >= 1.25.0 is required
     @classmethod
@@ -147,27 +156,6 @@ def _get_scopes_kwargs(
 
         return scopes_kwargs
 
-    # TODO: Remove this function once google-api-core >= 1.26.0 is required
-    @classmethod
-    def _get_self_signed_jwt_kwargs(
-        cls, host: str, scopes: Optional[Sequence[str]]
-    ) -> Dict[str, Union[Optional[Sequence[str]], str]]:
-        """Returns kwargs to pass to grpc_helpers.create_channel depending on the google-api-core version"""
-
-        self_signed_jwt_kwargs: Dict[str, Union[Optional[Sequence[str]], str]] = {}
-
-        if _API_CORE_VERSION and (
-            packaging.version.parse(_API_CORE_VERSION)
-            >= packaging.version.parse("1.26.0")
-        ):
-            self_signed_jwt_kwargs["default_scopes"] = cls.AUTH_SCOPES
-            self_signed_jwt_kwargs["scopes"] = scopes
-            self_signed_jwt_kwargs["default_host"] = cls.DEFAULT_HOST
-        else:
-            self_signed_jwt_kwargs["scopes"] = scopes or cls.AUTH_SCOPES
-
-        return self_signed_jwt_kwargs
-
     def _prep_wrapped_messages(self, client_info):
         # Precompute the wrapped methods.
         self._wrapped_methods = {
diff --git a/google/cloud/billing/budgets_v1/services/budget_service/transports/grpc.py b/google/cloud/billing/budgets_v1/services/budget_service/transports/grpc.py
index 4f2f143..d5d6975 100644
--- a/google/cloud/billing/budgets_v1/services/budget_service/transports/grpc.py
+++ b/google/cloud/billing/budgets_v1/services/budget_service/transports/grpc.py
@@ -61,6 +61,7 @@ def __init__(
         client_cert_source_for_mtls: Callable[[], Tuple[bytes, bytes]] = None,
         quota_project_id: Optional[str] = None,
         client_info: gapic_v1.client_info.ClientInfo = DEFAULT_CLIENT_INFO,
+        always_use_jwt_access: Optional[bool] = False,
     ) -> None:
         """Instantiate the transport.
 
@@ -101,6 +102,8 @@ def __init__(
                 API requests. If ``None``, then default info will be used.
                 Generally, you only need to set this if you're developing
                 your own client library.
+            always_use_jwt_access (Optional[bool]): Whether self signed JWT should
+                be used for service account credentials.
 
         Raises:
           google.auth.exceptions.MutualTLSChannelError: If mutual TLS transport
@@ -153,6 +156,7 @@ def __init__(
             scopes=scopes,
             quota_project_id=quota_project_id,
             client_info=client_info,
+            always_use_jwt_access=always_use_jwt_access,
         )
 
         if not self._grpc_channel:
@@ -208,14 +212,14 @@ def create_channel(
               and ``credentials_file`` are passed.
         """
 
-        self_signed_jwt_kwargs = cls._get_self_signed_jwt_kwargs(host, scopes)
-
         return grpc_helpers.create_channel(
             host,
             credentials=credentials,
             credentials_file=credentials_file,
             quota_project_id=quota_project_id,
-            **self_signed_jwt_kwargs,
+            default_scopes=cls.AUTH_SCOPES,
+            scopes=scopes,
+            default_host=cls.DEFAULT_HOST,
             **kwargs,
         )
 
diff --git a/google/cloud/billing/budgets_v1/services/budget_service/transports/grpc_asyncio.py b/google/cloud/billing/budgets_v1/services/budget_service/transports/grpc_asyncio.py
index 542a31e..f0e77c6 100644
--- a/google/cloud/billing/budgets_v1/services/budget_service/transports/grpc_asyncio.py
+++ b/google/cloud/billing/budgets_v1/services/budget_service/transports/grpc_asyncio.py
@@ -82,14 +82,14 @@ def create_channel(
             aio.Channel: A gRPC AsyncIO channel object.
         """
 
-        self_signed_jwt_kwargs = cls._get_self_signed_jwt_kwargs(host, scopes)
-
         return grpc_helpers_async.create_channel(
             host,
             credentials=credentials,
             credentials_file=credentials_file,
             quota_project_id=quota_project_id,
-            **self_signed_jwt_kwargs,
+            default_scopes=cls.AUTH_SCOPES,
+            scopes=scopes,
+            default_host=cls.DEFAULT_HOST,
             **kwargs,
         )
 
@@ -107,6 +107,7 @@ def __init__(
         client_cert_source_for_mtls: Callable[[], Tuple[bytes, bytes]] = None,
         quota_project_id=None,
         client_info: gapic_v1.client_info.ClientInfo = DEFAULT_CLIENT_INFO,
+        always_use_jwt_access: Optional[bool] = False,
     ) -> None:
         """Instantiate the transport.
 
@@ -148,6 +149,8 @@ def __init__(
                 API requests. If ``None``, then default info will be used.
                 Generally, you only need to set this if you're developing
                 your own client library.
+            always_use_jwt_access (Optional[bool]): Whether self signed JWT should
+                be used for service account credentials.
 
         Raises:
             google.auth.exceptions.MutualTlsChannelError: If mutual TLS transport
@@ -199,6 +202,7 @@ def __init__(
             scopes=scopes,
             quota_project_id=quota_project_id,
             client_info=client_info,
+            always_use_jwt_access=always_use_jwt_access,
         )
 
         if not self._grpc_channel:
diff --git a/google/cloud/billing/budgets_v1beta1/services/budget_service/transports/base.py b/google/cloud/billing/budgets_v1beta1/services/budget_service/transports/base.py
index af26665..a533ec9 100644
--- a/google/cloud/billing/budgets_v1beta1/services/budget_service/transports/base.py
+++ b/google/cloud/billing/budgets_v1beta1/services/budget_service/transports/base.py
@@ -24,6 +24,7 @@
 from google.api_core import gapic_v1  # type: ignore
 from google.api_core import retry as retries  # type: ignore
 from google.auth import credentials as ga_credentials  # type: ignore
+from google.oauth2 import service_account  # type: ignore
 
 from google.cloud.billing.budgets_v1beta1.types import budget_model
 from google.cloud.billing.budgets_v1beta1.types import budget_service
@@ -47,8 +48,6 @@
     except pkg_resources.DistributionNotFound:  # pragma: NO COVER
         _GOOGLE_AUTH_VERSION = None
 
-_API_CORE_VERSION = google.api_core.__version__
-
 
 class BudgetServiceTransport(abc.ABC):
     """Abstract transport class for BudgetService."""
@@ -69,6 +68,7 @@ def __init__(
         scopes: Optional[Sequence[str]] = None,
         quota_project_id: Optional[str] = None,
         client_info: gapic_v1.client_info.ClientInfo = DEFAULT_CLIENT_INFO,
+        always_use_jwt_access: Optional[bool] = False,
         **kwargs,
     ) -> None:
         """Instantiate the transport.
@@ -92,6 +92,8 @@ def __init__(
                 API requests. If ``None``, then default info will be used.
                 Generally, you only need to set this if you're developing
                 your own client library.
+            always_use_jwt_access (Optional[bool]): Whether self signed JWT should
+                be used for service account credentials.
         """
         # Save the hostname. Default to port 443 (HTTPS) if none is specified.
         if ":" not in host:
@@ -101,7 +103,7 @@ def __init__(
         scopes_kwargs = self._get_scopes_kwargs(self._host, scopes)
 
         # Save the scopes.
-        self._scopes = scopes or self.AUTH_SCOPES
+        self._scopes = scopes
 
         # If no credentials are provided, then determine the appropriate
         # defaults.
@@ -120,13 +122,20 @@ def __init__(
                 **scopes_kwargs, quota_project_id=quota_project_id
             )
 
+        # If the credentials is service account credentials, then always try to use self signed JWT.
+        if (
+            always_use_jwt_access
+            and isinstance(credentials, service_account.Credentials)
+            and hasattr(service_account.Credentials, "with_always_use_jwt_access")
+        ):
+            credentials = credentials.with_always_use_jwt_access(True)
+
         # Save the credentials.
         self._credentials = credentials
 
-    # TODO(busunkim): These two class methods are in the base transport
+    # TODO(busunkim): This method is in the base transport
     # to avoid duplicating code across the transport classes. These functions
-    # should be deleted once the minimum required versions of google-api-core
-    # and google-auth are increased.
+    # should be deleted once the minimum required versions of google-auth is increased.
 
     # TODO: Remove this function once google-auth >= 1.25.0 is required
     @classmethod
@@ -147,27 +156,6 @@ def _get_scopes_kwargs(
 
         return scopes_kwargs
 
-    # TODO: Remove this function once google-api-core >= 1.26.0 is required
-    @classmethod
-    def _get_self_signed_jwt_kwargs(
-        cls, host: str, scopes: Optional[Sequence[str]]
-    ) -> Dict[str, Union[Optional[Sequence[str]], str]]:
-        """Returns kwargs to pass to grpc_helpers.create_channel depending on the google-api-core version"""
-
-        self_signed_jwt_kwargs: Dict[str, Union[Optional[Sequence[str]], str]] = {}
-
-        if _API_CORE_VERSION and (
-            packaging.version.parse(_API_CORE_VERSION)
-            >= packaging.version.parse("1.26.0")
-        ):
-            self_signed_jwt_kwargs["default_scopes"] = cls.AUTH_SCOPES
-            self_signed_jwt_kwargs["scopes"] = scopes
-            self_signed_jwt_kwargs["default_host"] = cls.DEFAULT_HOST
-        else:
-            self_signed_jwt_kwargs["scopes"] = scopes or cls.AUTH_SCOPES
-
-        return self_signed_jwt_kwargs
-
     def _prep_wrapped_messages(self, client_info):
         # Precompute the wrapped methods.
         self._wrapped_methods = {
diff --git a/google/cloud/billing/budgets_v1beta1/services/budget_service/transports/grpc.py b/google/cloud/billing/budgets_v1beta1/services/budget_service/transports/grpc.py
index 7464528..b08226d 100644
--- a/google/cloud/billing/budgets_v1beta1/services/budget_service/transports/grpc.py
+++ b/google/cloud/billing/budgets_v1beta1/services/budget_service/transports/grpc.py
@@ -61,6 +61,7 @@ def __init__(
         client_cert_source_for_mtls: Callable[[], Tuple[bytes, bytes]] = None,
         quota_project_id: Optional[str] = None,
         client_info: gapic_v1.client_info.ClientInfo = DEFAULT_CLIENT_INFO,
+        always_use_jwt_access: Optional[bool] = False,
     ) -> None:
         """Instantiate the transport.
 
@@ -101,6 +102,8 @@ def __init__(
                 API requests. If ``None``, then default info will be used.
                 Generally, you only need to set this if you're developing
                 your own client library.
+            always_use_jwt_access (Optional[bool]): Whether self signed JWT should
+                be used for service account credentials.
 
         Raises:
           google.auth.exceptions.MutualTLSChannelError: If mutual TLS transport
@@ -153,6 +156,7 @@ def __init__(
             scopes=scopes,
             quota_project_id=quota_project_id,
             client_info=client_info,
+            always_use_jwt_access=always_use_jwt_access,
         )
 
         if not self._grpc_channel:
@@ -208,14 +212,14 @@ def create_channel(
               and ``credentials_file`` are passed.
         """
 
-        self_signed_jwt_kwargs = cls._get_self_signed_jwt_kwargs(host, scopes)
-
         return grpc_helpers.create_channel(
             host,
             credentials=credentials,
             credentials_file=credentials_file,
             quota_project_id=quota_project_id,
-            **self_signed_jwt_kwargs,
+            default_scopes=cls.AUTH_SCOPES,
+            scopes=scopes,
+            default_host=cls.DEFAULT_HOST,
             **kwargs,
         )
 
diff --git a/google/cloud/billing/budgets_v1beta1/services/budget_service/transports/grpc_asyncio.py b/google/cloud/billing/budgets_v1beta1/services/budget_service/transports/grpc_asyncio.py
index e15b051..c03e1d0 100644
--- a/google/cloud/billing/budgets_v1beta1/services/budget_service/transports/grpc_asyncio.py
+++ b/google/cloud/billing/budgets_v1beta1/services/budget_service/transports/grpc_asyncio.py
@@ -82,14 +82,14 @@ def create_channel(
             aio.Channel: A gRPC AsyncIO channel object.
         """
 
-        self_signed_jwt_kwargs = cls._get_self_signed_jwt_kwargs(host, scopes)
-
         return grpc_helpers_async.create_channel(
             host,
             credentials=credentials,
             credentials_file=credentials_file,
             quota_project_id=quota_project_id,
-            **self_signed_jwt_kwargs,
+            default_scopes=cls.AUTH_SCOPES,
+            scopes=scopes,
+            default_host=cls.DEFAULT_HOST,
             **kwargs,
         )
 
@@ -107,6 +107,7 @@ def __init__(
         client_cert_source_for_mtls: Callable[[], Tuple[bytes, bytes]] = None,
         quota_project_id=None,
         client_info: gapic_v1.client_info.ClientInfo = DEFAULT_CLIENT_INFO,
+        always_use_jwt_access: Optional[bool] = False,
     ) -> None:
         """Instantiate the transport.
 
@@ -148,6 +149,8 @@ def __init__(
                 API requests. If ``None``, then default info will be used.
                 Generally, you only need to set this if you're developing
                 your own client library.
+            always_use_jwt_access (Optional[bool]): Whether self signed JWT should
+                be used for service account credentials.
 
         Raises:
             google.auth.exceptions.MutualTlsChannelError: If mutual TLS transport
@@ -199,6 +202,7 @@ def __init__(
             scopes=scopes,
             quota_project_id=quota_project_id,
             client_info=client_info,
+            always_use_jwt_access=always_use_jwt_access,
         )
 
         if not self._grpc_channel:
diff --git a/noxfile.py b/noxfile.py
index 70417e8..03aa2f5 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -179,7 +179,7 @@ def docs(session):
     """Build the docs for this library."""
 
     session.install("-e", ".")
-    session.install("sphinx", "alabaster", "recommonmark")
+    session.install("sphinx==4.0.1", "alabaster", "recommonmark")
 
     shutil.rmtree(os.path.join("docs", "_build"), ignore_errors=True)
     session.run(
@@ -201,7 +201,9 @@ def docfx(session):
     """Build the docfx yaml files for this library."""
 
     session.install("-e", ".")
-    session.install("sphinx", "alabaster", "recommonmark", "gcp-sphinx-docfx-yaml")
+    session.install(
+        "sphinx==4.0.1", "alabaster", "recommonmark", "gcp-sphinx-docfx-yaml"
+    )
 
     shutil.rmtree(os.path.join("docs", "_build"), ignore_errors=True)
     session.run(
diff --git a/setup.py b/setup.py
index 1c6499e..a3be920 100644
--- a/setup.py
+++ b/setup.py
@@ -22,10 +22,10 @@
 
 name = "google-cloud-billing-budgets"
 description = "Cloud Billing Budget API API client library"
-version = "1.2.0"
+version = "1.3.0"
 release_status = "Development Status :: 4 - Beta"
 dependencies = [
-    "google-api-core[grpc] >= 1.22.2, < 2.0.0dev",
+    "google-api-core[grpc] >= 1.26.0, <2.0.0dev",
     "proto-plus >= 1.10.0",
     "packaging >= 14.3",
 ]
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
index fd8f2bc..9edacb4 100644
--- a/testing/constraints-3.6.txt
+++ b/testing/constraints-3.6.txt
@@ -5,7 +5,7 @@
 #
 # e.g., if setup.py has "foo >= 1.14.0, < 2.0.0dev",
 # Then this file should have foo==1.14.0
-google-api-core==1.22.2
+google-api-core==1.26.0
 proto-plus==1.10.0
 libcst==0.2.5
 packaging==14.3
diff --git a/tests/unit/gapic/budgets_v1/test_budget_service.py b/tests/unit/gapic/budgets_v1/test_budget_service.py
index 7ce7bee..0696cc4 100644
--- a/tests/unit/gapic/budgets_v1/test_budget_service.py
+++ b/tests/unit/gapic/budgets_v1/test_budget_service.py
@@ -37,9 +37,6 @@
 from google.cloud.billing.budgets_v1.services.budget_service import BudgetServiceClient
 from google.cloud.billing.budgets_v1.services.budget_service import pagers
 from google.cloud.billing.budgets_v1.services.budget_service import transports
-from google.cloud.billing.budgets_v1.services.budget_service.transports.base import (
-    _API_CORE_VERSION,
-)
 from google.cloud.billing.budgets_v1.services.budget_service.transports.base import (
     _GOOGLE_AUTH_VERSION,
 )
@@ -53,8 +50,9 @@
 import google.auth
 
 
-# TODO(busunkim): Once google-api-core >= 1.26.0 is required:
-# - Delete all the api-core and auth "less than" test cases
+# TODO(busunkim): Once google-auth >= 1.25.0 is required transitively
+# through google-api-core:
+# - Delete the auth "less than" test cases
 # - Delete these pytest markers (Make the "greater than or equal to" tests the default).
 requires_google_auth_lt_1_25_0 = pytest.mark.skipif(
     packaging.version.parse(_GOOGLE_AUTH_VERSION) >= packaging.version.parse("1.25.0"),
@@ -65,16 +63,6 @@
     reason="This test requires google-auth >= 1.25.0",
 )
 
-requires_api_core_lt_1_26_0 = pytest.mark.skipif(
-    packaging.version.parse(_API_CORE_VERSION) >= packaging.version.parse("1.26.0"),
-    reason="This test requires google-api-core < 1.26.0",
-)
-
-requires_api_core_gte_1_26_0 = pytest.mark.skipif(
-    packaging.version.parse(_API_CORE_VERSION) < packaging.version.parse("1.26.0"),
-    reason="This test requires google-api-core >= 1.26.0",
-)
-
 
 def client_cert_source_callback():
     return b"cert bytes", b"key bytes"
@@ -137,6 +125,36 @@ def test_budget_service_client_from_service_account_info(client_class):
         assert client.transport._host == "billingbudgets.googleapis.com:443"
 
 
+@pytest.mark.parametrize(
+    "client_class", [BudgetServiceClient, BudgetServiceAsyncClient,]
+)
+def test_budget_service_client_service_account_always_use_jwt(client_class):
+    with mock.patch.object(
+        service_account.Credentials, "with_always_use_jwt_access", create=True
+    ) as use_jwt:
+        creds = service_account.Credentials(None, None, None)
+        client = client_class(credentials=creds)
+        use_jwt.assert_not_called()
+
+
+@pytest.mark.parametrize(
+    "transport_class,transport_name",
+    [
+        (transports.BudgetServiceGrpcTransport, "grpc"),
+        (transports.BudgetServiceGrpcAsyncIOTransport, "grpc_asyncio"),
+    ],
+)
+def test_budget_service_client_service_account_always_use_jwt_true(
+    transport_class, transport_name
+):
+    with mock.patch.object(
+        service_account.Credentials, "with_always_use_jwt_access", create=True
+    ) as use_jwt:
+        creds = service_account.Credentials(None, None, None)
+        transport = transport_class(credentials=creds, always_use_jwt_access=True)
+        use_jwt.assert_called_once_with(True)
+
+
 @pytest.mark.parametrize(
     "client_class", [BudgetServiceClient, BudgetServiceAsyncClient,]
 )
@@ -1905,7 +1923,6 @@ def test_budget_service_transport_auth_adc_old_google_auth(transport_class):
         (transports.BudgetServiceGrpcAsyncIOTransport, grpc_helpers_async),
     ],
 )
-@requires_api_core_gte_1_26_0
 def test_budget_service_transport_create_channel(transport_class, grpc_helpers):
     # If credentials and host are not provided, the transport class should use
     # ADC credentials.
@@ -1937,82 +1954,6 @@ def test_budget_service_transport_create_channel(transport_class, grpc_helpers):
         )
 
 
-@pytest.mark.parametrize(
-    "transport_class,grpc_helpers",
-    [
-        (transports.BudgetServiceGrpcTransport, grpc_helpers),
-        (transports.BudgetServiceGrpcAsyncIOTransport, grpc_helpers_async),
-    ],
-)
-@requires_api_core_lt_1_26_0
-def test_budget_service_transport_create_channel_old_api_core(
-    transport_class, grpc_helpers
-):
-    # If credentials and host are not provided, the transport class should use
-    # ADC credentials.
-    with mock.patch.object(
-        google.auth, "default", autospec=True
-    ) as adc, mock.patch.object(
-        grpc_helpers, "create_channel", autospec=True
-    ) as create_channel:
-        creds = ga_credentials.AnonymousCredentials()
-        adc.return_value = (creds, None)
-        transport_class(quota_project_id="octopus")
-
-        create_channel.assert_called_with(
-            "billingbudgets.googleapis.com:443",
-            credentials=creds,
-            credentials_file=None,
-            quota_project_id="octopus",
-            scopes=(
-                "https://www.googleapis.com/auth/cloud-billing",
-                "https://www.googleapis.com/auth/cloud-platform",
-            ),
-            ssl_credentials=None,
-            options=[
-                ("grpc.max_send_message_length", -1),
-                ("grpc.max_receive_message_length", -1),
-            ],
-        )
-
-
-@pytest.mark.parametrize(
-    "transport_class,grpc_helpers",
-    [
-        (transports.BudgetServiceGrpcTransport, grpc_helpers),
-        (transports.BudgetServiceGrpcAsyncIOTransport, grpc_helpers_async),
-    ],
-)
-@requires_api_core_lt_1_26_0
-def test_budget_service_transport_create_channel_user_scopes(
-    transport_class, grpc_helpers
-):
-    # If credentials and host are not provided, the transport class should use
-    # ADC credentials.
-    with mock.patch.object(
-        google.auth, "default", autospec=True
-    ) as adc, mock.patch.object(
-        grpc_helpers, "create_channel", autospec=True
-    ) as create_channel:
-        creds = ga_credentials.AnonymousCredentials()
-        adc.return_value = (creds, None)
-
-        transport_class(quota_project_id="octopus", scopes=["1", "2"])
-
-        create_channel.assert_called_with(
-            "billingbudgets.googleapis.com:443",
-            credentials=creds,
-            credentials_file=None,
-            quota_project_id="octopus",
-            scopes=["1", "2"],
-            ssl_credentials=None,
-            options=[
-                ("grpc.max_send_message_length", -1),
-                ("grpc.max_receive_message_length", -1),
-            ],
-        )
-
-
 @pytest.mark.parametrize(
     "transport_class",
     [
@@ -2035,10 +1976,7 @@ def test_budget_service_grpc_transport_client_cert_source_for_mtls(transport_cla
             "squid.clam.whelk:443",
             credentials=cred,
             credentials_file=None,
-            scopes=(
-                "https://www.googleapis.com/auth/cloud-billing",
-                "https://www.googleapis.com/auth/cloud-platform",
-            ),
+            scopes=None,
             ssl_credentials=mock_ssl_channel_creds,
             quota_project_id=None,
             options=[
@@ -2145,10 +2083,7 @@ def test_budget_service_transport_channel_mtls_with_client_cert_source(transport
                 "mtls.squid.clam.whelk:443",
                 credentials=cred,
                 credentials_file=None,
-                scopes=(
-                    "https://www.googleapis.com/auth/cloud-billing",
-                    "https://www.googleapis.com/auth/cloud-platform",
-                ),
+                scopes=None,
                 ssl_credentials=mock_ssl_cred,
                 quota_project_id=None,
                 options=[
@@ -2195,10 +2130,7 @@ def test_budget_service_transport_channel_mtls_with_adc(transport_class):
                 "mtls.squid.clam.whelk:443",
                 credentials=mock_cred,
                 credentials_file=None,
-                scopes=(
-                    "https://www.googleapis.com/auth/cloud-billing",
-                    "https://www.googleapis.com/auth/cloud-platform",
-                ),
+                scopes=None,
                 ssl_credentials=mock_ssl_cred,
                 quota_project_id=None,
                 options=[
diff --git a/tests/unit/gapic/budgets_v1beta1/test_budget_service.py b/tests/unit/gapic/budgets_v1beta1/test_budget_service.py
index 09a8b0c..35bec6c 100644
--- a/tests/unit/gapic/budgets_v1beta1/test_budget_service.py
+++ b/tests/unit/gapic/budgets_v1beta1/test_budget_service.py
@@ -39,9 +39,6 @@
 )
 from google.cloud.billing.budgets_v1beta1.services.budget_service import pagers
 from google.cloud.billing.budgets_v1beta1.services.budget_service import transports
-from google.cloud.billing.budgets_v1beta1.services.budget_service.transports.base import (
-    _API_CORE_VERSION,
-)
 from google.cloud.billing.budgets_v1beta1.services.budget_service.transports.base import (
     _GOOGLE_AUTH_VERSION,
 )
@@ -55,8 +52,9 @@
 import google.auth
 
 
-# TODO(busunkim): Once google-api-core >= 1.26.0 is required:
-# - Delete all the api-core and auth "less than" test cases
+# TODO(busunkim): Once google-auth >= 1.25.0 is required transitively
+# through google-api-core:
+# - Delete the auth "less than" test cases
 # - Delete these pytest markers (Make the "greater than or equal to" tests the default).
 requires_google_auth_lt_1_25_0 = pytest.mark.skipif(
     packaging.version.parse(_GOOGLE_AUTH_VERSION) >= packaging.version.parse("1.25.0"),
@@ -67,16 +65,6 @@
     reason="This test requires google-auth >= 1.25.0",
 )
 
-requires_api_core_lt_1_26_0 = pytest.mark.skipif(
-    packaging.version.parse(_API_CORE_VERSION) >= packaging.version.parse("1.26.0"),
-    reason="This test requires google-api-core < 1.26.0",
-)
-
-requires_api_core_gte_1_26_0 = pytest.mark.skipif(
-    packaging.version.parse(_API_CORE_VERSION) < packaging.version.parse("1.26.0"),
-    reason="This test requires google-api-core >= 1.26.0",
-)
-
 
 def client_cert_source_callback():
     return b"cert bytes", b"key bytes"
@@ -139,6 +127,36 @@ def test_budget_service_client_from_service_account_info(client_class):
         assert client.transport._host == "billingbudgets.googleapis.com:443"
 
 
+@pytest.mark.parametrize(
+    "client_class", [BudgetServiceClient, BudgetServiceAsyncClient,]
+)
+def test_budget_service_client_service_account_always_use_jwt(client_class):
+    with mock.patch.object(
+        service_account.Credentials, "with_always_use_jwt_access", create=True
+    ) as use_jwt:
+        creds = service_account.Credentials(None, None, None)
+        client = client_class(credentials=creds)
+        use_jwt.assert_not_called()
+
+
+@pytest.mark.parametrize(
+    "transport_class,transport_name",
+    [
+        (transports.BudgetServiceGrpcTransport, "grpc"),
+        (transports.BudgetServiceGrpcAsyncIOTransport, "grpc_asyncio"),
+    ],
+)
+def test_budget_service_client_service_account_always_use_jwt_true(
+    transport_class, transport_name
+):
+    with mock.patch.object(
+        service_account.Credentials, "with_always_use_jwt_access", create=True
+    ) as use_jwt:
+        creds = service_account.Credentials(None, None, None)
+        transport = transport_class(credentials=creds, always_use_jwt_access=True)
+        use_jwt.assert_called_once_with(True)
+
+
 @pytest.mark.parametrize(
     "client_class", [BudgetServiceClient, BudgetServiceAsyncClient,]
 )
@@ -1553,7 +1571,6 @@ def test_budget_service_transport_auth_adc_old_google_auth(transport_class):
         (transports.BudgetServiceGrpcAsyncIOTransport, grpc_helpers_async),
     ],
 )
-@requires_api_core_gte_1_26_0
 def test_budget_service_transport_create_channel(transport_class, grpc_helpers):
     # If credentials and host are not provided, the transport class should use
     # ADC credentials.
@@ -1585,82 +1602,6 @@ def test_budget_service_transport_create_channel(transport_class, grpc_helpers):
         )
 
 
-@pytest.mark.parametrize(
-    "transport_class,grpc_helpers",
-    [
-        (transports.BudgetServiceGrpcTransport, grpc_helpers),
-        (transports.BudgetServiceGrpcAsyncIOTransport, grpc_helpers_async),
-    ],
-)
-@requires_api_core_lt_1_26_0
-def test_budget_service_transport_create_channel_old_api_core(
-    transport_class, grpc_helpers
-):
-    # If credentials and host are not provided, the transport class should use
-    # ADC credentials.
-    with mock.patch.object(
-        google.auth, "default", autospec=True
-    ) as adc, mock.patch.object(
-        grpc_helpers, "create_channel", autospec=True
-    ) as create_channel:
-        creds = ga_credentials.AnonymousCredentials()
-        adc.return_value = (creds, None)
-        transport_class(quota_project_id="octopus")
-
-        create_channel.assert_called_with(
-            "billingbudgets.googleapis.com:443",
-            credentials=creds,
-            credentials_file=None,
-            quota_project_id="octopus",
-            scopes=(
-                "https://www.googleapis.com/auth/cloud-billing",
-                "https://www.googleapis.com/auth/cloud-platform",
-            ),
-            ssl_credentials=None,
-            options=[
-                ("grpc.max_send_message_length", -1),
-                ("grpc.max_receive_message_length", -1),
-            ],
-        )
-
-
-@pytest.mark.parametrize(
-    "transport_class,grpc_helpers",
-    [
-        (transports.BudgetServiceGrpcTransport, grpc_helpers),
-        (transports.BudgetServiceGrpcAsyncIOTransport, grpc_helpers_async),
-    ],
-)
-@requires_api_core_lt_1_26_0
-def test_budget_service_transport_create_channel_user_scopes(
-    transport_class, grpc_helpers
-):
-    # If credentials and host are not provided, the transport class should use
-    # ADC credentials.
-    with mock.patch.object(
-        google.auth, "default", autospec=True
-    ) as adc, mock.patch.object(
-        grpc_helpers, "create_channel", autospec=True
-    ) as create_channel:
-        creds = ga_credentials.AnonymousCredentials()
-        adc.return_value = (creds, None)
-
-        transport_class(quota_project_id="octopus", scopes=["1", "2"])
-
-        create_channel.assert_called_with(
-            "billingbudgets.googleapis.com:443",
-            credentials=creds,
-            credentials_file=None,
-            quota_project_id="octopus",
-            scopes=["1", "2"],
-            ssl_credentials=None,
-            options=[
-                ("grpc.max_send_message_length", -1),
-                ("grpc.max_receive_message_length", -1),
-            ],
-        )
-
-
 @pytest.mark.parametrize(
     "transport_class",
     [
@@ -1683,10 +1624,7 @@ def test_budget_service_grpc_transport_client_cert_source_for_mtls(transport_cla
             "squid.clam.whelk:443",
             credentials=cred,
             credentials_file=None,
-            scopes=(
-                "https://www.googleapis.com/auth/cloud-billing",
-                "https://www.googleapis.com/auth/cloud-platform",
-            ),
+            scopes=None,
             ssl_credentials=mock_ssl_channel_creds,
             quota_project_id=None,
             options=[
@@ -1793,10 +1731,7 @@ def test_budget_service_transport_channel_mtls_with_client_cert_source(transport
                 "mtls.squid.clam.whelk:443",
                 credentials=cred,
                 credentials_file=None,
-                scopes=(
-                    "https://www.googleapis.com/auth/cloud-billing",
-                    "https://www.googleapis.com/auth/cloud-platform",
-                ),
+                scopes=None,
                 ssl_credentials=mock_ssl_cred,
                 quota_project_id=None,
                 options=[
@@ -1843,10 +1778,7 @@ def test_budget_service_transport_channel_mtls_with_adc(transport_class):
                 "mtls.squid.clam.whelk:443",
                 credentials=mock_cred,
                 credentials_file=None,
-                scopes=(
-                    "https://www.googleapis.com/auth/cloud-billing",
-                    "https://www.googleapis.com/auth/cloud-platform",
-                ),
+                scopes=None,
                 ssl_credentials=mock_ssl_cred,
                 quota_project_id=None,
                 options=[