The conditions for obtaining runaway electron (RE) current pulses with a minimum duration at a subnanosecond breakdown of centimeter gaps filled with air at atmospheric pressure were studied. It is shown that the RE current pulse duration depends on many parameters, such as the cathode form, the magnitude of the interelectrode distance, the diameter of the diaphragm, which is part of the anode, as well as on the size of the receiving part of the collector. It has been established that the use of cathodes with an extended edge having a small radius of curvature, RE current pulses consisting of two peaks of picosecond duration can be recorded.