Logarithmic Temperature Dependence of the Conductivity and Lack of   Universal One-Parameter Scaling in the Two-Dimensional Metal
  We show that the two-dimensional metallic state in Si-MOS samples persists over a wide range of temperatures (16 mK to 8 K), sample peak mobilities (varying by a factor of 8), carrier densities (0.8 to $35\times 10^{11}$ cm$^{-2}$) and conductances from 0.3 to $120 e^2/h$. Our data reveal a failure of the universal one-parameter scaling. We have found a weak delocalizing logarithmic correction to the conductivity, which governs a weak increase in the conductivity of the 2D metal as $T$ approaches zero. 
V. M. Pudalov, G. Brunthaler, A. Prinz, G. Bauer