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Shot noise in the edge states of 2D topological insulators

We calculate the resistance and shot noise in the edge states of a 2D topological insulator that result from the exchange of electrons between these states and conducting puddles in the bulk of the insulator. The two limiting cases where the energy relaxation is either absent or very strong are considered. A finite time of spin relaxation in the puddles is introduced phenomenologically. Depending on this time and on the strength of coupling between the edge states and the puddles, the Fano factor $F=S_I/2eI$ ranges from 0 to 1/3, which is in an agreement with the available experimental data.