  We develop a two-channel resistor model for simulating spin transport with
general applicability. Using this model, for the case of graphene as a
prototypical material, we calculate the spin signal consistent with
experimental values. Using the same model we also simulate the charge and spin-
dependent 1/f noise, both in the local and nonlocal four-probe measurement
schemes, and identify the noise from the spin-relaxation resistances as the
major source of spin-dependent 1/f noise.
