I have a 54 cm Soma Buena Vista mixte. The actual "top tube" height (from the floor) is 21 1/2 inches. http://www.somafab.com/archives/product/buena-vista

If you are thinking less than 24 inches would work for you, this might be a direction to look.

It is a fast road-worthy bike. I find it to be a stiffer and faster ride than my Surly with identical parts.

The frame set (frame and fork) is about $550. The cheapest way to get the pieces and parts you need to build it up is to buy a used bike and swap everything onto the Soma frame. That's what I did. And that's how I know the Soma is faster than my Surly. They truly have identical parts (except for the brakes). Only the frames are different.

My hips have good days and bad days. When I'm not in the mood to swing my right leg over the rear wheel and saddle, doing the step-through routine on the Soma Buena Vista works nicely. It also saves some grief when the rear rack is fully loaded and awkward to swing a leg around.