  Spin-flips are one of the limiting factors for spin-based information
processing. We demonstrate a transport approach for determining the spin-flip
rates of a self-assembled InAs double quantum dot occupied by a single
electron. In such devices, different Land\'e factors lead to an inhomogeneous
Zeeman splitting, so that the two spin channels can never be at resonance
simultaneously, leading to a spin blockade at low temperatures. This blockade
is analyzed in terms of spin flips for different temperatures and magnetic
fields. Our results are in good agreement with a quantum master equation that
combines the dot-lead couplings with ohmic dissipation stemming from spin-flip
cotunneling.
