Does a plea agreement where the government's promise of leniency is contingent upon truthful testimony at trial violate due process?

Plea agreements wherein the government's promise of leniency is contingent upon a person's truthful testimony at trial do not violate due process. See United States v. Yarbrough, 852 F.2d 1522, 1537-38 (9th Cir.1988) (contingent plea agreement did not violate due process where leniency offered in exchange for truthful testimony and beneficiary cross-examined regarding agreement); see also United States v. Moody, 778 F.2d 1380, 1385 (9th Cir.1985), amended on other grounds, 791 F.2d 707 (9th Cir.1986) (plea agreements often contingent upon agreement that beneficiary testify truthfully against others).