  In this letter we address the question how interactions affect the DC
conductance of a one-dimensional electron system not necessarily adequately
described by the Luttinger model. Using a Laughlin type argument, we show that
gauge invariance protects the universal value of the conductance of $e^2/h$ per
channel per spin orientation if the system possesses two conserved charges
conjugate to the chemical potentials of the external reservoirs.
