What's This Cable Connecting Cockpit to Vertical Stabilizer in Air Tractor AT-401?
The wire shown is a cable deflector which is part of a Wire-Strike Protection System common on most agricultural aircraft.

Wire-strike protection systems
As a last resort, when pilot situational awareness and on-board systems fail to detect a wire in sufficient time to avoid contact, a passive wirestrike protection system (WSPS) may protect the aircraft from the consequences of a wire-strike. These are designed to cut or deflect wires away from an aircraft. The types of WSPS vary depending on whether the aircraft is fixed-wing or rotary-wing. They have proven to be an effective safeguard by extensive testing and over two decades of use by both military and non-military operators' worldwide (Jackson, Boitnott, Fasanella, Jones &amp; Lyle, 2004; RAAF, 1 997). However, to enable the WSPS to operate effectively, the wire must contact the cutter at an appropriate angle and the aircraft must also have adequate forward speed. This combination of circumstances may not always be present during low-level aerial operations.
WSPS for fixed-wing aircraft are designed to cut wires that could pass under the aircraft, in order to prevent the wires from coming into contact with the landing gear, or pass over the aircraft, possibly contacting the tail section. Serrated deflection wires may also be fitted from the cabin to the tail section, with the purpose of cutting the wire or lifting it over the tail section. Fixed-wing aircraft used in aerial agriculture operations have had WSPS fitted as standard equipment for several years.
Fixed-wing aircraft used for other purposes rarely carry WSPS as fitment often requires reinforcing parts of the aircraft and may cost several thousand dollars. In general, they are not fitted unless it is expected that the aircraft will spend many hours in low-level flight.