  The mechanisms limiting the spin coherence time of electrons are of great
importance for spintronics. We present electron spin resonance (ESR) and
transport measurements of six different two dimensional electron gases in
silicon/silicon-germanium (Si/SiGe 2DEGs). The spin decoherence time $T_2^*$ is
presented in conjunction with the 2DEG density $n_e$ and momentum scattering
time $\tau_p$ as measured from transport experiments. A pronounced dependence
of $T_2^*$ on the orientation of the applied magnetic field with respect to
2DEG layer is found which is not consistent with that expected from any
mechanism described in the literature.
