Universality of equilibrium one-dimensional transport from gauge invariance

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.