A note on the nucleation with multiple steps: Parallel and series   nucleation
  Parallel and series nucleation are the basic elements of the complex nucleation process when two saddle points exist on the free-energy landscape. It is pointed out that the nucleation rates follow formulas similar to those of parallel and series connection of resistors or conductors in an electric circuit. Necessary formulas to calculate individual nucleation rates at the saddle points and the total nucleation rate are summarized and the extension to the more complex nucleation process is suggested. 
Masao Iwamatsu
cond-mat.mtrl-sci cond-mat.stat-mech