  Detailed comparisons are reported between laboratory observations of
electron-scale dissipation layers near a reconnecting X-line and direct
two-dimensional full-particle simulations. Many experimental features of the
electron layers, such as insensitivity to the ion mass, are reproduced by the
simulations; the layer thickness, however, is about 3-5 times larger than the
predictions. Consequently, the leading candidate 2D mechanism based on
collisionless electron nongyrotropic pressure is insufficient to explain the
observed reconnection rates. These results suggest that, in addition to the
residual collisions, 3D effects play an important role in electron-scale
dissipation during fast reconnection.
