What is the test for impeaching a witness?

"Materiality . . . requires more than a showing that the suppressed evidence would have been admissible [citation], that the absence of the suppressed evidence made conviction 'more likely' [citation], or that using the suppressed evidence to discredit a witness's testimony 'might have changed the outcome of the trial' [citation]. A defendant instead 'must show a "reasonable probability of a different result." ' " (People v. Salazar, supra, 35 Cal.4th at p. 1043.)

" 'In general, impeachment evidence has been found to be material where the witness at issue "supplied the only evidence linking the defendant(s) to the crime," [citations], or where the likely impact on the witness's credibility would have undermined a critical element of the prosecution's case, [citations]. In contrast, a new trial is generally not required when the testimony of the witness "is corroborated by other testimony." ' " (People v. Salazar, supra, 35 Cal.4th at p. 1050.)