  Effective-medium theory is applied to the percolation description of the
metal-insulator transition in two dimensions with emphasis on the continuous
connection between the zero-magnetic-field transition and the quantum Hall
transition. In this model the system consists of puddles connected via saddle
points, and there is loss of quantum coherence inside the puddles. The
effective conductance of the network is calculated using appropriate
integration over the distribution of conductances, leading to a determination
of the magnetic field dependence of the critical density. Excellent
quantitative agreement is obtained with the experimental data, which allows an
estimate of the puddle physical parameters.
