  The valley splitting, which lifts the degeneracy of the lowest two valley
states in a SiO$_2$/(100)Si/SiO$_2$ quantum well is examined through transport
measurements. We demonstrate that the valley splitting can be observed directly
as a step in the conductance defining a boundary between valley-unpolarized and
polarized regions. This persists to well above liquid helium temperature and
shows no dependence on magnetic field, indicating that single-particle valley
splitting and valley-polarization exist in (100) silicon even at zero magnetic
field.
