What is the test for establishing that misconduct was a direct or exclusive cause?

own behalf. To do so, he is not required to prove that the conduct under challenge was the 'direct or exclusive' cause. [Citations.] Rather, he need only show that the conduct was a substantial cause. [Citations.] The misconduct in question may be deemed a substantial cause when, for example, it carries significant coercive force [citation] and is soon followed by the witness's refusal to testify [citation]." (Ibid.) Finally, the defendant must demonstrate a reasonable possibility exists that the witness's testimony would have been both material and favorable to his defense. (Id. at p. 32; see People v. Lucas (1995) 12 Cal.4th 415, 457.)