  We report novel manifestation of the valley splitting for the two valley
electron system in (100) Si-inversion layers at low carrier density. We found
that valley splitting causes almost 100% modulation of the Shubnikov de Haas
oscillations in very low magnetic fields, almost on the bound of the quantum
interference peak of the negative magnetoresistance. From the interference
pattern of oscillations we determined the valley splitting in the B=0 limit
which appears to vary only within a factor of 1.3 over the density range
(3-7)x10^{11}/cm^2. We found also that level broadenings in both electron
valleys differ only by < 3%. The latter result shows that the inter-valley
scattering is not responsible for the strong (six fold) `metallic-like' changes
of the resistivity with temperature.
