  In conventional metals, electronic transport in a magnetic field is
characterized by the motion of electrons along orbits on the Fermi surface,
which usually causes an increase in the resistivity through averaging over
velocities. Here we show that large deviations from this behavior can arise in
density-wave systems close to their ordering temperature. Specifically,
enhanced scattering off collective fluctuations can lead to a change of
direction of the orbital motion on reconstructed pockets. In weak magnetic
fields, this leads to linear magnetoconductivity, the sign of which depends on
the electric-field direction. At a critical magnetic field, the conductivity
crosses zero for certain directions, signifying a thermodynamic instability of
the density-wave state.
