  We apply a two-particle semi-analytic approach to a single Anderson impurity
attached to two biased metallic leads. The theory is based on reduced parquet
equations justified in critical regions of singularities in the Bethe-Salpeter
equations. It offers a way to treat one-particle and two-particle thermodynamic
and spectral quantities on the same footing. The two-particle vertices are
appropriately renormalized so that spurious transitions into the magnetic state
of the weak-coupling approximations are suppressed. The unphysical hysteresis
loop in the current-voltage characteristics is thereby eliminated. Furthermore,
in the linear response regime, we qualitatively reproduce the three transport
regimes with the increasing temperature: from the Kondo resonant tunneling
through the Coulomb blockade regime up to a sequential tunneling regime. Far
from equilibrium, we find that the bias plays a similar role as the temperature
in destroying the Kondo resonant peak when the corresponding energy scale is
comparable with the Kondo temperature. Besides that, the applied voltage in low
bias is shown to develop spectral peaks around the lead chemical potentials as
observed in previous theoretical and experimental studies.
