  We find a renormalized "time-dependent diffusion coefficient", D(t), for
pulsed excitation of a nominally diffusive sample by solving the Bethe-Salpeter
equation with recurrent scattering. We observe a crossover in dynamics in the
transformation from a quasi-1D to a slab geometry implemented by varying the
ratio of the radius, R, of the cylindrical sample with reflecting walls and the
sample length, L. Immediately after the peak of the transmitted pulse, D(t)
falls linearly with a nonuniversal slope that approaches an asymptotic value
for R/L >> 1. The value of D(t) extrapolated to t = 0 depends only upon the
dimensionless conductance, g, for R/L << l 1 and only upon kl for R/L >> 1,
where k is the wave vector and l is the bare mean free path.
