  Simulations suggest collisionless steady-state magnetic reconnection of
Harris-type current sheets proceeds with a rate of order 0.1, independent of
dissipation mechanism. We argue this long-standing puzzle is a result of
constraints at the magnetohydrodynamic (MHD) scale. We perform a scaling
analysis of the reconnection rate as a function of the opening angle made by
the upstream magnetic fields, finding a maximum reconnection rate close to 0.2.
The predictions compare favorably to particle-in-cell simulations of
relativistic electron-positron and non-relativistic electron-proton
reconnection. The fact that simulated reconnection rates are close to the
predicted maximum suggests reconnection proceeds near the most efficient state
allowed at the MHD-scale. The rate near the maximum is relatively insensitive
to the opening angle, potentially explaining why reconnection has a similar
fast rate in differing models.
