  Antisymmetrized molecular dynamics with quantum branching is generalized so
as to allow finite time duration of the unrestricted coherent mean field
propagation which is followed by the decoherence into wave packets. In this new
model, the wave packet shrinking by the mean field propagation is respected as
well as the diffusion, so that it predicts a one-body dynamics similar to that
in mean field models. The shrinking effect is expected to change the diffusion
property of nucleons in nuclear matter and the global one-body dynamics. The
central $\xenon+\tin$ collisions at 50 MeV/nucleon are calculated by the models
with and without shrinking, and it is shown that the inclusion of the wave
packet shrinking has a large effect on the multifragmentation in a big
expanding system with a moderate expansion velocity.
