  A qualitatively different character of dHvA oscillations has been found in a
multiband (quasi)two dimensional Fermi liquid with a fixed fermion density
$n_{e}$ (canonical ensemble) compared with an open system where the chemical
potential $\mu$ is kept fixed (grand canonical ensemble). A new fundamental
period $P_{f}$ appears when $n_{e}$ is fixed, a damping of the Landau levels is
relatively small and a background density of states is negligible. $P_{f}$ is
determined by the total density rather than by the partial densities of
carriers in different bands: $P_{f}=1/(2n_{e}\phi)$ for spin-split Landau
levels and $P_{f}=1/(n_{e}\phi)$ in the case of spin degenerate levels where
$\phi$ is the flux quantum.
