  The lifting of the two-fold degeneracy of the conduction valleys in a
strained silicon quantum well is critical for spin quantum computing. Here, we
obtain an accurate measurement of the splitting of the valley states in the
low-field region of interest, using the microwave spectroscopy technique of
electron valley resonance (EVR). We compare our results with conventional
methods, observing a linear magnetic field dependence of the valley splitting,
and a strong low-field suppression, consistent with recent theory. The
resonance linewidth shows a marked enhancement above $T\simeq 300$ mK.
