Ehrenfest Time and the Coherent Backscattering Off Ballistic Cavities

If the Ehrenfest time ₄ of a ballistic cavity is not negligible in comparison to its dwell time ₃, the weak localization correction to the cavity's transmission is suppressed proportional to exp (-₄/₃). At the same time, quantum interference enhances the probability of reflection into the mode of incidence by a factor two. This ``enhanced backscattering'' does not depend on the Ehrenfest time. We show that, in addition to the diagonal enhanced backscattering, there are off-diagonal contributions to coherent backscattering that become relevant if ₄₃.