Maximum Metallic Conductivity in Si-MOS Structures
  We found that the conductivity of the two-dimensional electron system in Si-MOS structures is limited to a maximum value, G_{max}, as either density increases or temperature decreases. This value G_{max} is weakly disorder dependent and ranging from 100 to 140 e^2/h for samples whose mobilities differ by a factor of 4. 
V. M. Pudalov, G. Brunthaler, A. Prinz, G. Bauer