Auth Type restriction for CredentialCache clss

The method 'Add' has a parameter for Authentication Type. Its values can be
'Basic', 'Digest', 'Negotiate', or 'Kerberos'. Two questions

a. If we set it to Kerberos, does it mean that it will use only Kerberos and
not other registered authentication?
b. Is there a way to ensure that it uses only Kerberos and doesn't fall down
to NTLM or other ways of authentication? We want the connection to fail if
Kerberos is not available.