  Since the experimental realization of graphene1, extensive theoretical work
has focused on short-range disorder2-5, ''ripples''6, 7, or charged
impurities2, 3, 8-13 to explain the conductivity as a function of carrier
density sigma_(n)[1,14-18], and its minimum value sigma_min near twice the
conductance quantum 4e2/h[14, 15, 19, 20]. Here we vary the density of charged
impurities nimp on clean graphene21 by deposition of potassium in ultra high
vacuum. At non-zero carrier density, charged impurity scattering produces the
ubiquitously observed1, 14-18 linear sigma_(n) with the theoretically-predicted
magnitude. The predicted asymmetry11 for attractive vs. repulsive scattering of
Dirac fermions is observed. Sigma_min occurs not at the carrier density which
neutralizes nimp, but rather the carrier density at which the average impurity
potential is zero10. Sigma_min decreases initially with nimp, reaching a
minimum near 4e2/h at non-zero nimp, indicating that Sigma_min in present
experimental samples does not probe Dirac-point physics14, 15, 19, 20 but
rather carrier density inhomogeneity due to the impurity potential3, 9, 10.
