AFAIK, regenerative braking is possible only with direct drive motors and the GMAC sold by ebikesca, and there are no mid-drives with it. It can be functional with a DD system, but some companies don't include it. IMO, it isn't a desirable feature since the amount of charge realized isn't enough to make up for the rapid change in motor "direction" (and effect on the motor and rear axle) when it's activated.