Two-dimensionally-quantized electron transport in modulation-doped double-heterojunction structures is investigated using a Monte Carlo simulation. Enhanced low-field mobility in the quantum well is observed. This results from the reduction of optical phonon scattering rates, which can be attributed to the spread of the two-dimensional electron gas. It is demonstrated that the carrier transport related to this enhancement and carrier confinement effectively contribute to device operation in a double-heterojunction FET.<<ETX>>