  We present a study of thermal conductivity in the normal state of the
heavy-fermion superconductor URu$_{2}$Si$_{2}$. Ordering at 18K leads to a
steep increase in thermal conductivity and (in contrast with all other cases of
magnetic ordering in heavy-fermion compounds) to an enhancement of the Lorenz
number. By linking this observation to several other previously reported
features, we conclude that most of the carriers disappear in the ordered state
and this leads to a drastic increase in both phononic and electronic
mean-free-path.
