  We find the nonlinear conductance of a dissipative resonant level in the
nonequilibrium steady state near its quantum critical point. The system
consists of a spin-polarized quantum dot connected to two resistive leads that
provide ohmic dissipation. We focus on the crossover from the strong-coupling,
non-Fermi-liquid regime to the weak-coupling, Fermi-liquid ground state, a
crossover driven by the instability of the quantum critical point to
hybridization asymmetry or detuning of the level in the dot. We show that the
crossover properties are given by tunneling through an effective single barrier
described by the boundary sine-Gordon model. The nonlinear conductance is then
obtained from thermodynamic Bethe ansatz results in the literature, which were
developed to treat tunneling in a Luttinger liquid. The current-voltage
characteristics are thus found for any value of the resistance of the leads.
For the special case of lead resistance equal to the quantum resistance, we
find mappings onto, first, the two-channel Kondo model and, second, an
effectively noninteracting model from which the nonlinear conductance is found
analytically. A key feature of the general crossover function is that the
nonequilibrium crossover driven by applied bias is different from the crossover
driven by temperature -- we find that the nonequilibrium crossover is
substantially sharper. Finally, we compare to experimental results for both the
bias and temperature crossovers: the agreement is excellent.
