  The minimal conductivity of graphene is a quantity measured in the DC limit.
It is shown, using the Kubo formula, that the actual value of the minimal
conductivity is sensitive to the order in which certain limits are taken. If
the DC limit is taken before the integration over energies is performed, the
minimal conductivity of graphene is $4/\pi$ (in units of $e^2/h$) and it is
$\pi/2$ in the reverse order. The value $\pi$ is obtained if weak disorder is
included via a small frequency-dependent selfenergy. In the high-frequency
limit the minimal conductivity approaches $\pi/2$ and drops to zero if the
frequency exceeds the cut-off energy of the particles.
