May a district court exclude a defense witness when he will invoke the Fifth Amendment in response to certain questions but undisputedly will answer at least one relevant and highly probative question?

The second question presented is whether an affidavit in support of a search warrant based on a confidential informant is sufficient under the Fourth Amendment when it fails to adequately demonstrate the informant's reliability?

The third question presented is whether a marijuana conviction in a state like Alabama may properly serve as the basis for the application of the career offender enhancement under the Sentencing Guidelines?

And the fourth question presented is whether the Sixth Amendment guarantee of the assistance of counsel protects a defendant's right to participate, in collaboration with his lawyer, in questioning and argument?

May a district court exclude a defense witness when he will invoke the Fifth Amendment in response to certain questions but undisputedly will answer at least one relevant and highly probative question?