פירוש על קידושין 129:21

The issue at hand in this mishnah is whether we think the husband betrothed his ex-wife when he stayed with her at an inn (the betrothal might have been done through sex). But again, were there witnesses? If there were, then why would Bet Shammai not require him to give her a get? And if there were no witnesses, why would Bet Hillel say she does require a get. Without witnesses, kiddushin are invalid.
Therefore, Rava again assumes there was one witness. And since Bet Hillel requires a get, this is a difficulty against R. Nahman.