  In the study of oscillatory electron transport in high-mobility Si MOSFETs at
low temperatures we observed two correlated effects in weak in-plane magnetic
fields: a steep decrease of the renormalized magnetic susceptibility
$\chi^*(H)$ and an increase of the concentration of mobile carriers $n(H)$. We
suggest a phenomenological model of the magnetic field driven redistribution
between the extended and localized electronic states that qualitatively
explains both effects. We argue that the redistribution is mainly caused by
magnetization of the large-spin localized states with energies close to the
Fermi energy $E_F$, coexisting with the majority Fermi liquid state. Our
findings also resolve a long-standing disagreement between the experimental
data on $\chi^*$ obtained in weak ($H \sim k_BT/\mu_B$) and strong ($H \sim
E_F/g\mu_B$) magnetic fields.
