2D Metal-Insulator transition as a percolation transition
  By carefully analyzing the low temperature density dependence of 2D conductivity in undoped high mobility n-GaAs heterostructures, we conclude that the 2D metal-insulator transition in this system is a density inhomogeneity driven percolation transition due to the breakdown of screening in the random charged impurity disorder background. In particular, our measured conductivity exponent of $\sim 1.4$ approaches the 2D percolation exponent value of 4/3 at low temperatures and our experimental data are inconsistent with there being a zero-temperature quantum critical point in our system. 
S. Das Sarma (1,2), M. P. Lilly (2), E. H. Hwang (1), L. N. Pfeiffer   (3), K. W. West (3), J. L. Reno (2) ((1) Condensed Ma...