  Using fully kinetic simulations, we study the scaling of the inflow speed of
collisionless magnetic reconnection from the non-relativistic to
ultra-relativistic limit. In the anti-parallel configuration, the inflow speed
increases with the upstream magnetization parameter $\sigma$ and approaches the
light speed when $\sigma > O(100)$, leading to an enhanced reconnection rate.
In all regimes, the divergence of pressure tensor is the dominant term
responsible for breaking the frozen-in condition at the x-line. The observed
scaling agrees well with a simple model that accounts for the Lorentz
contraction of the plasma passing through the diffusion region. The results
demonstrate that the aspect ratio of the diffusion region remains $\sim 0.1$ in
both the non-relativistic and relativistic limits.
