Novel Properties of The Apparent Metal-Insulator Transition in   Two-Dimensional Systems
  The low-temperature conductivity of low-density, high-mobility, two-dimensional hole systems in GaAs was studied. We explicitly show that the metal-insulator transition, observed in these systems, is characterized by a well-defined critical density, p_0c. We also observe that the low-temperature conductivity of these systems depends linearly on the hole density, over a wide density range. The high-density linear conductivity extrapolates to zero at a density close to the critical density. 
Y. Hanein, D. Shahar, J. Yoon, C.C. Li, D.C. Tsui and Hadas Shtrikman