  Relaxation process of a coherent scalar field oscillation in the thermal bath
is investigated using nonequilibrium quantum field theory. The Langevin-type
equation of motion is obtained which has a memory term and both additive and
multiplicative noise terms. The dissipation rate of the oscillating scalar
field is calculated for various interactions such as Yukawa coupling,
three-body scalar interaction, and biquadratic interaction. When the background
temperature is larger than the oscillation frequency, the dissipation rate
arising from the interactions with fermions is suppressed due to the Pauli
blocking, while it is enhanced for interactions with bosons due to the induced
effect. In both cases, we find that the microphysical detailed balance relation
drives the oscillating field to a thermal equilibrium state. That is, for
low-momentum modes, the classical fluctuation-dissipation theorem holds and
they relax to a state the equipartition law is satisfied, while higher-momentum
modes reach the state the number density of each quanta consists of the thermal
boson distribution function and zero-point vacuum contribution. The
temperature-dependent dissipation rates obtained here are applied to the late
reheating phase of inflationary universe. It is found that in some cases the
reheat temperature may take somewhat different value from the conventional
estimates, and in an extreme case the inflaton can dissipate its energy without
linear interactions that leads to its decay. Furthermore the evaporation rate
of the Affleck-Dine field at the onset of its oscillation is calculated.
