  We develop a low-energy nonequilibrium field theory for weakly interacting
quantum dots. The theory is based on the Keldysh field integral in the spin
channel of the quantum dot described by the single impurity Anderson
Hamiltonian. The effective Keldysh action is a functional of the
Hubbard-Stratonovich magnetization field decoupling the quantum dot spin
channel. We expand this action up to the second order with respect to the
magnetization field, which allows to describe nonequilibrium interacting
quantum dots at low temperatures and weak electron-electron interactions,up to
the contacts-dot coupling energy. Besides its simplicity, an additional
advantage of the theory is that it correctly describes the unitary limit giving
the correct result for the conductance maximum. Thus our theory establishes an
alternative simple method relevant for investigation of weakly interacting
nonequilibrium nanodevices.
