  The electrical current through an arbitrary junction connecting quantum wires
of spinless interacting fermions is calculated in fermionic representation. The
wires are adiabatically attached to two reservoirs at chemical potentials
differing by the applied voltage bias. The relevant scale-dependent
contributions in perturbation theory in the interaction up to infinite order
are evaluated and summed up. The result allows one to construct renormalization
group equations for the conductance as a function of voltage (or temperature,
wire length). There are two fixed points at which the conductance follows a
power law in terms of a scaling variable $\Lambda$, which equals the bias
voltage $V$, if $V$ is the largest energy scale compared to temperature $T$ and
inverse wire length $L^{-1}$, and interpolates between these quantities in the
crossover regimes.
