  We derive an exact expression for the Kubo conductunce in the Quantum Hall
device with the point-like intra-edge backscattering. This involves the
calculation of current-current correlator exactly, which we perform using
form-factor method. In brief, the full set of intermediate states is inserted
in the correlator, and for each term the closed mathematical expression is
obtained. It is shown that by making a special choice of intermediate states in
accordance with the hidden symmetries of the model, one achieves fast
convergence of the series, thus proving the form-factor approach to be
especially powerfull. This review is based on the joint work with H.Saleur and
F.Lesage.
