  In high-purity crystals of YBa_2Cu_3O_7, the quasiparticle (qp) lifetime
  \tau and the (weak-field) thermal Hall conductivity \kappa_{xy} undergo
dramatic increases below 90K. We present a detailed picture of the behavior of
\kappa_{xy} at low temperature, in particular its scaling properties, which are
directly relevant to the issue of whether Landau quantization of the qp states
occurs.
