Tunneling Through Sphalerons: The O(3) Sigma-Model on a Cylinder
  We construct all instantons for the \nlsig\ on a cylinder, known not to exist on a finite time interval. We show that the widest instantons go through sphalerons. A re-interpretation of moduli-space transforms the scale parameter $\rho$ to a boundary condition in time. This may give a handle on the $\rho\rightarrow0$ divergent instanton gas. 
Jeroen Snippe