Electron transport under a large amplitude RF field in GaxIn1-xSb has been simulated by the Monte Carlo method, to examine the validity of the assumption usually employed in the microwave heating experiment to obtain the velocity-field curve. The result indicates that a large relaxation time from the upper to the lower valley, about 35 ps for x=0.7, makes it invalid to assume an instantaneous electron response to the applied microwave field at 35 GHz. It also shows that an apparent velocity-field curve with negative differential mobility characteristics may result if the conventional procedure is applied, neglecting the relaxation effect.