  The transport properties of a bilayer graphene are studied theoretically
within a self-consistent Born approximation. The electronic spectrum is
composed of $k$-linear dispersion in the low-energy region and $k$-square
dispersion as in an ordinary two-dimensional metal at high energy, leading to a
crossover between different behaviors in the conductivity on changing the Fermi
energy or disorder strengths. We find that the conductivity approaches
$2e^2/\pi^2\hbar$ per spin in the strong-disorder regime, independently of the
short- or long-range disorder.
