  We (re) consider in this paper the problem of tunneling through an impurity
in a quantum wire with arbitrary Luttinger interaction parameter. By combining
the integrable approach developed in the case of Quantum Hall edge states with
the introduction of radiative boundary conditions to describe the adiabatic
coupling to reservoirs, we are able to obtain the exact equilibrium and non
equilibrium current. One of the most striking features observed is the
appearance of negative differential conductances out of equilibrium in the
strongly interacting regime g <=.2. In spite of the various charging effects, a
remarkable form of duality is still observed.
  New results on the computation of transport properties in integrable impurity
problems are gathered in appendices. In particular, we prove that the TBA
results satisfy a remarkable relation, originally derived using the Keldysh
formalism, between the order T^2 correction to the current out of equilibrium
and the second derivative of this current at T=0 with respect to the voltage.
