  We discuss the residual carrier density (n*) near the Dirac point (DP) in
graphene estimated by quantum capacitance (CQ) and conductivity measurements.
The CQ at the DP has a finite value and is independent of the temperature. A
similar behavior is also observed for the conductivity at the DP, because their
origin is residual carriers induced externally by charged impurities. The n*
extracted from CQ, however, is often smaller than that from the conductivity,
suggesting that the mobility in the puddle region is lower than that in the
linear region. The CQ measurement should be employed for estimating n*
quantitatively.
