  We compute the DC conductance with two different methods, which both exploit
the integrability of the theories under consideration. On one hand we determine
the conductance through a defect by means of the thermodynamic Bethe ansatz and
standard relativistic potential scattering theory based on a Landauer transport
theory picture. On the other hand, we propose a Kubo formula for a defect
system and evaluate the current-current two-point correlation function it
involves with the help of a form factor expansion. For a variety of defects in
a fermionic system we find excellent agreement between the two different
theoretical descriptions.
