  We report the observation of counterflowing edge current in InAs quantum
wells which leads to the breakdown of quantum Hall (QH) effects at high
magnetic fields. Counterflowing edge channels arise from the Fermi-level
pinning of InAs and the resultant sharp edge potential with downward bending.
By measuring the counterflow conductance for varying edge lengths, we determine
the effective number $\langle N_\text{C} \rangle$ of counterflowing modes and
their equilibration length $\lambda_\text{eq}$ at bulk integer filling factor
$\nu = 1$--$4$. $\lambda_\text{eq}$ increased exponentially with magnetic field
$B$, reaching $200~\mu$m for $\nu = 4$ at $B \geq 7.6$~T. Our data reveal
important roles of the innermost incompressible strip with even filling in
determining $\langle N_\text{C} \rangle$ and $\lambda_\text{eq}$ and the impact
of the contact configuration on the QH effect breakdown. Our results show that
counterflowing edge channels manifest as transport anomalies only at high
fields and in short edges. This in turn suggests that, even in the integer QH
regime, the actual microscopic structure of edge states can differ from that
anticipated from macroscopic transport measurements, which is relevant to
various systems including atomic-layer materials.
