For plugs, i use Amphenol KS3PB-AU (3.5mm jack TRS) and Amphenol ACPS-GN (6.35mm jack TRS).
Between those, i use Sommer Cable SC-Goblin White (the total external diameter is 4.6 mm). It is a patch cable, 2 signals + ground, rather flexible. The 4.6mm diameter is the max i think is comfortable to solder into a mini jack, yet big enough to fit in the 6.35mm plug's strain relief.
(here is some picture of the mini jack end)