  Measurements of longitudinal and transverse transport coefficients were
performed in the normal state of a high-quality single crystal of
optimally-doped superconductor YBa2Cu3O6.95. We found the Hall-Lorenz number
(Lxy) depends only weakly on temperature, thus no appreciable violation of the
Wiedemann-Franz law has been observed. However, Lxy is about two times larger
than the Sommerfeld's value of the Lorenz number. Our results can be
interpreted consistently if we assume that the electronic system of
YBa2Cu3O6.95 behaves like a Fermi-liquid with a pseudo-gap that opens at the
Fermi level.
