  We reexamine basic aspects of a nonequilibrium steady state in the Kondo
problem for a quantum dot under a bias voltage using a reduced density matrix,
which is obtained in the Fock space by integrating out one of the two
conduction channels. The integration has been carried out by discretizing the
conduction channels preserving the two-fold degeneracy due to the left-going
and right-going scattering states. The remaining subspace is described by a
single-channel Anderson model, and the statistical weight is determined by the
reduced density matrix. In the noninteracting case, it can be constructed as
the mixed states that show a close similarity to the high-temperature
distribution in equilibrium. Specifically, if the system has an inversion
symmetry, the one-particle states in an energy window between the two chemical
potentials \mu_R and \mu_L are occupied, or unoccupied, completely at random
with an equal weight. The Coulomb interaction preserves these aspects, and the
correlation functions can be expressed in a Lehmann-representation form using
the mixed-state statistical weight.

Strongly Correlated Electrons;Mesoscale and Nanoscale Physics