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Recently, a single-layer of carbon atoms, termed graphene, has attracted a great deal of interest due to its great potential for application in electronics. In experiments involving graphene, a finite residual conductivity was found at zero gate voltage in the density dependence of conductivity. However, the theoretical explanation of this observation has been confused, with derivations predicting differing values of residual conductivity. In this paper, considering electron-impurity scattering, we present a kinetic equation approach to investigate transport in graphene. The effect of interband polarization on conductivity is taken into account. We find that, in the density dependence of conductivity, there is a minimum (rather than residual) conductivity sensitively dependent on the carrier-impurity scattering potential. For higher electron density, the conductivity varies almost linearly with the electron density.