Entropy in Black Hole Pair Production

Pair production of Reissner-Nordstrom black holes in a magnetic field can be described by a euclidean instanton. It is shown that the instanton amplitude contains an explicit factor of $e^{A\mathrm{/}4}$, where $A$ is the area of the event horizon. This is consistent with the hypothesis that $e^{A\mathrm{/}4}$ measures the number of black hole states.