Motion of fast particles in strongly fluctuating magnetic fields

The problem of phase coherence of a fermion in a spin liquid state of a quantum magnet is reduced to the problem of phase coherence of a fermion moving in a random magnetic field. The latter problem is solved for the fermions moving with large velocity, i.e., close to the Fermi surface. The motion of such fermions turns out to be classical in the direction perpendicular to the classical trajectory, but it remains quantum in the direction along the trajectory.