What is the test for a defendant's Sixth Amendment right to access counsel?

The right to the assistance of counsel "was designed to assure fairness in the adversary criminal process.... [In other words,] the purpose of providing assistance of counsel 'is simply to ensure that criminal defendants receive a fair trial,' [citation].... [I]n evaluating Sixth Amendment claims, 'the appropriate inquiry focuses on the adversarial process, not on the accused's relationship with his lawyer as such.' [Citation.] Thus, while the right to select and be represented by one's preferred attorney is comprehended by the Sixth Amendment, the essential aim of the Amendment is to guarantee an effective advocate for each criminal defendant rather than to ensure that a defendant will inexorably be represented by the lawyer whom he prefers." (Wheat v. United States (1988) 486 U.S. 153, ---- - ----, 108 S.Ct. 1692, 1696-1697, 100 L.Ed.2d 140.)