We study contention-resolution protocols for multiple-access channels. We show that every backoff protocol is transient if the arrival rate, , is at least 0.42 and that the capacity of every backoff protocol is at most 0.42. Thus, we show that backoff protocols have (provably) smaller capacity than full-sensing protocols. Finally, we show that the corresponding results, with the larger arrival bound of 0.531, also hold for every acknowledgement-based protocol. This work was partially supported by the EPSRC grant "Sharper Analysis of Randomised Algorithms: a Computational Approach", NSF Grant CCR9820885, and the IST Programme of the EU under contract numbers IST-1999-14186 (ALCOM-FT) and IST-1999-14036 (RAND-APX). http://www.dcs.warwick.ac.uk/leslie/, Department of Computer Science, University of Warwick, Coventry, CV4 7AL, United Kingdom. http://www.dcs.ed.ac.uk/mrj/, Division of Informatics, University of Edinburgh, JCMB, The King's Buildings, Edinburgh EH9 3JZ, Uni...