  Theory of quasiparticle transport in the mixed state of a d-wave
superconductor is developed under the assumption of disordered vortex array. A
novel universal regime is identified at fields above H*= c*H_{c2}(T/T_c)^2,
characterized by a field-independent longitudinal thermal conductivity. It is
argued that this behavior is responsible for the high-field plateau in the
thermal conductivity experimentally observed in cuprates by Krishana, Ong and
co-workers.
