Coherence time in high energy proton-nucleus collisions

Precisely measured Drell-Yan cross sections for 800 GeV protons incident on a variety of nuclear targets exhibit a deviation from linear scaling in th e atomic number A. We show that this deviation can be accounted for by ener gy degradation of the proton as it passes through the nucleus if account is taken of the time delay of particle production due to quantum coherence. W e infer an average proper coherence time of 0.4 +/- 0.1 fm/c, corresponding to a coherence path length of 8 +/- 2 fm in the rest frame of the nucleus.