  A Kramers pair of helical edge states in quantum spin Hall effect (QSHE) is
robust against normal dephasing but not robust to spin dephasing. In our work,
we provide an effective spin dephasing mechanism in the puddles of
two-dimensional (2D) QSHE, which is simulated as quantum dots modeled by 2D
massive Dirac Hamiltonian. We demonstrate that the spin dephasing effect can
originate from the combination of the Rashba spin-orbit coupling and
electron-phonon interaction, which gives rise to inelastic backscattering in
edge states within the topological insulator quantum dots, although the
time-reversal symmetry is preserved throughout. Finally, we discuss the
tunneling between extended helical edge states and local edge states in the QSH
quantum dots, which leads to backscattering in the extended edge states. These
results can explain the more robust edge transport in InAs/GaSb QSH systems.
