Precisely measured Drell-Yan cross sections for 800 GeV protons incident on a
variety of nuclear targets exhibit a deviation from linear scaling in the
atomic number A. We show that this deviation can be accounted for by energy
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. We 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.Comment: 11 pages in LaTeX. Includes 6 eps figures. Uses epsf.st
