  The paper deals with the nonequilibrium two-lead Anderson model, considered
as an adequate description for transport through a d-c biased quantum dot.
Using a self-consistent equation-of-motion method generalized out of
equilibrium, we calculate a fourth-order decoherence rate $\gamma^{(4)}$
induced by a bias voltage $V$. This decoherence rate provides a cut-off to the
infrared divergences of the self-energy showing up in the Kondo regime. At low
temperature, the Kondo peak in the density of states is split into two peaks
pinned at the chemical potential of the two leads. The height of these peaks is
controlled by $\gamma^{(4)}$. The voltage dependence of the differential
conductance exhibits a zero-bias peak followed by a broad Coulomb peak at large
$V$, reflecting charge fluctuations inside the dot. The low-bias differential
conductance is found to be a universal function of the normalized bias voltage
$V/T_K$, where $T_K$ is the Kondo temperature. The universal scaling with a
single energy scale $T_K$ at low bias voltages is also observed for the
renormalized decoherence rate $\gamma^{(4)}/T_K$. We discuss the effect of
$\gamma^{(4)}$ on the crossover from strong to weak coupling regime when either
the temperature or the bias voltage is increased.
