  We measure the dependence of the conductivity of graphene as a function of
magnetic field, temperature and carrier density and discover a saturation of
the dephasing length at low temperatures that we ascribe to spin memory
effects. Values of the spin coherence length up to eight microns are found to
scale with the mean free path. We consider different origins of this effect and
suggest that it is controlled by resonant states that act as magnetic-like
defects. By varying the level of disorder, we demonstrate that the spin
coherence length can be tuned over an order of magnitude.
