  The Wiedemann-Franz (WF) ratio compares the thermal and electrical
conductivities in a metal. We describe a new way to determine its value, based
on the thermal Hall conductivity. The technique is applied to copper and to
untwinned YBaCuO. In the latter, we uncover a T-linear dependence and
suppression of the Hall-channel WF ratio. We discuss the implications of this
suppressions. The general suppression of the WF ratio in systems with
predominant electron-electron scattering is discussed.
