  We have measured the weak localization magnetoresistance in (001)-oriented Si
MOS structures with a wide range of mobilities. For the quantitative analysis
of the data, we have extended the theory of weak-localization corrections in
the ballistic regime to the system with two equivalent valleys in electron
spectrum. This theory describes the observed magnetoresistance and allows the
extraction of the phase breaking time tau_phi and the intervalley scattering
time tau_v. The temperature dependences tau_phi(T) for all studied structures
are in good agreement with the theory of electron-electron interaction effects
in two-dimensional systems. The intervalley scattering is elastic and rather
strong: tau_v is typically only an order of magnitude greater than the
transport time, tau. It is found that the intervalley scattering rate is
temperature-independent and the ratio tau_v/tau decreases with increasing the
electron density. These observations suggest that the roughness of the Si-SiO2
interface plays the major role in intervalley scattering.
