where σ represents the spin up/down electrons. G/G † indicates the retarted/advanced Green's function of the central region. The coupling of central region to the electrodes is taken into account using self-energies, which are calculated by imposing a wide-band limit approximation which assumes a constant density of states, independent of energy. A constant value of 0.036 ev -1 , taken from literature is used for the DOS of electrode. The zero bias conductance in CSS limit is obtained as