Landau quantisation
 
Landau quantization is the quantization of the cyclotron orbits of charged particles in magnetic fields.  
As a result, the charged particles can only occupy orbits with discrete energy values, called Landau levels.  
The Landau levels are degenerate, with the number of electrons per level directly proportional to the strength of the applied magnetic field.  
 
Source:
http://en.wikipedia.org/wiki/Landau_quantization