  We have measured the conductivity of high-mobility (001) Si
metal-oxide-semiconductor field effect transistors (MOSFETs) over wide ranges
of electron densities n=(1.8-15)x10^11cm^2, temperatures T=30mK-4.2K, and
in-plane magnetic fields B=0-5T. The experimental data have been analyzed using
the theory of interaction effects in the conductivity of disordered 2D systems.
The parameters essential for comparison with the theory, such as the
intervalley scattering time and valley splitting, have been measured or
evaluated in independent experiments. The observed behavior of the
conductivity, including its quasi-linear increase with decreasing T down to
~0.4K and its downturn at lower temperatures, is in agreement with the theory.
The values of the Fermi- liquid parameter obtained from the comparison agree
with the corresponding values extracted from the analysis of Shubnikov-de Haas
oscillations based on the theory of magnetooscillations in interacting 2D
systems.
