  We analyze universal transport properties of a strongly interacting quantum
dot in the Kondo regime when the quantum dot is placed in an external magnetic
field. The quantum dot is described by the asymmetric Anderson model with the
spin degeneracy removed by the magnetic field resulting in the Zeeman
splitting. Using an analytical expression for the tunneling density of states
found from a Keldysh effective field theory, we obtain in the whole energy
range the universal differential conductance and analytically demonstrate its
Fermi-liquid and logarithmic behavior at low- and high-energies, respectively,
as a function of the magnetic field. We also show results on the zero
temperature differential conductance as a function of the bias voltage at
different magnetic fields as well as results on finite temperature effects out
of equilibrium and at a finite magnetic field. The modern nonequilibrium
experimental issues of the critical magnetic field, at which the zero bias
maximum of the differential conductance starts to split into two maxima, as
well as the distance between these maxima as a function of the magnetic field
are also addressed.
