  We calculate the dissipation rate of a coherently oscillating scalar field in
a thermal environment using nonequilibrium quantum field theory and apply it to
the reheating stage after cosmic inflation. It is shown that the rate is
nonvanishing even when particles coupled to the oscillating inflaton field have
a larger thermal mass than it, and therefore the cosmic temperature can be much
higher than inflaton's mass even in the absence of preheating. Its cosmological
implications are also discussed.
