  The wave function for the matter field fluctuations in the infrared sector is
studied within the framework of inflationary cosmology. These fluctuations are
described by a coarse-grained field which takes into account only the modes
with wavelength much bigger than the size of the Hubble horizon. The case of a
power-law expanding universe is considered and it is found that the relevant
phase-space remains coherent under certain circumstances. In this case the
classical stochastic treatment for matter field fluctuations is not valid,
however, for $p > 4.6$, the system loses its coherence and a classical
stochastic approximation is allowed.
