  Cyclotron resonance of two-dimensional electrons is studied for a
high-mobility Si/SiGe quantum well in the presence of an in-plane magnetic
field, which induces spin polarization. The relaxation time $\tau_{CR}$ shows a
negative in-plane magnetic field dependence, which is similar to that of the
transport scattering time $\tau_t$ obtained from dc resistivity. The resonance
magnetic field shows an unexpected negative shift with increasing in-plane
magnetic field.
