  We investigate quantum dynamics and kinetics of a 2D conductor with closed
Fermi surface reconstructed by a biaxial density wave, in which electrons move
along a two-dimensional periodic net of semiclassical trajectories coupled by
the magnetic breakdown tunnelling under a strong magnetic field. We derive a
quasi-particle dispersion law and magnetoconductivity tensor. The
quasi-particle spectrum is found to be the alternating series of
two-dimensional magnetic energy bands with gaps between them. The longitudinal
magnetoconductivity shows giant oscillations with change of magnetic field,
while the Hall coefficient changes sign and is absent in a wide range of the
magnetic fields in between. Preliminary estimations show that the suggested
magnetoconductivity mechanism may be the origin of such behaviour of the Hall
coefficient vs. magnetic field, as observed in experiments in materials with
analogous topology of the Fermi surface, such as the high-Tc superconducting
cuprates.
