Composition Adjustment Teleport

A Composition Adjustment Teleport (C.A.T) was a type of transmat. By slowing down or speeding up the movement of electrons, the gap between the electrons is widened. The C.A.T used this gap to shrink and enlarge objects. Onboard Space Station Alpha, a Genesis III used a C.A.T to miniaturise humans and have them live on a world the size of a carbon atom.

The Fourth Doctor was aware of the mechanics and regarded them to be toys. (DWA: Inter-Galactic Cat)