  We present the theory and measurement of valley splitting in a quantum point
contact (QPC) in a modulation doped Si/SiGe heterostructure. Our measurements
are performed on a submicron Schottky-gated device. An effective mass theory is
developed for a QPC formed in a quantum well, grown on a miscut substrate. Both
theory and experiments include a perpendicular magnetic field. Our results
indicate that both QPC and magnetic confinement can enhance the valley
splitting by reducing the spatial extent of the electronic wavefunction.
Consequently, the valley splitting can be much larger than the spin splitting
for small magnetic fields. We also observe different valley splittings for
different transverse modes in the QPC, supporting the notion that when steps
are present at the quantum well interface, the spatial extent of the
wavefunction plays a dominant role in determining the valley splitting.
