  We have studied the preheating phase for a class of plateau inflationary
model considering the four-legs interaction term $(1/2)g^2\phi^2\chi^2$ between
the inflaton $(\phi)$ and reheating field $(\chi)$. We specifically focus on
the effects of a parameter $\phi_*$ that controls inflationary dynamics and the
shape of the inflaton potential. For $\phi_* < M_p$, the departure of the
inflaton potential from the usual power-law behavior $\phi^n$ significantly
modifies the microscopic behavior of the preheating dynamics. We analyze and
compare the efficiency of production, thermalization and the final equation of
state of the system for different models under consideration with $n=2,4,6$ for
two different values of $\phi_*$. Most importantly as we increase $n$, or
decrease $\phi_*$, the preheating occurs very efficiently with the final
equation of state to be that of the radiation, $w=1/3$. Specially for $n=2$,
the final equation of state turned out to be $w\simeq 0.2$. However, a complete
decay of inflaton could not be achieved with the four-legs interaction for any
model under consideration. Therefore, in order to complete the reheating
process, we perform the perturbative analysis for the second stage of the
reheating phase. Taking the end product of the preheating phase as an initial
condition we have solved the homogeneous Boltzmann equations for both the
fields supplemented by the constraints coming from the subsequent entropy
conservation. In so doing, we can calculate the reheating temperature which is
otherwise ill-defined right after the end of preheating. The temperature can be
uniquely fixed for a given inflaton decay constant and the CMB temperature. We
also compare our results with the conventional reheating constraint analysis
and discuss the limit of inflaton decay constant from the field theory
perspective.
