We have grown n-channel Si:SiGe quantum wells using gas source molecular beam epitaxy. The two-dimensional electron gas confined in the strained Si quantum well is weakly disordered and displays quantum interference in the form of a pronounced, low-field negative magnetoresistance. By fitting the data to the standard theory of weak localization, in the temperature range 50 mK to 6 K, we have been able to extract the product N,u and the electron phase coherence length L,. Our results give a value for the product N,u in the range 0.6 0.12, which is close to that expected for strong intervalley scattering and is similar to values obtained in Si MOSFETs. We show a power law dependence of L, with temperature, i.e. L, cc T-Pl2. For T > 0.7 K the value of the exponent p/2 = 0.75 which suggests that the dominant dephasing mechanism is electron-phonon scattering as observed in Si MOSFETs. However, for lower temperatures the variation is less pronounced (p!2 = 0.15). The weaker temperature dependence may be a precursor to a saturation in the value of L, as has recently been observed by other groups