Calculating Characteristic Impedance of Jumper Wire
Impedance is  \$Z_o=\sqrt{L/C}\$ with twisted pair being about 220 ohms with tolerance depending on wire radius~gap and 30~40 twists per meter. 

For just a wire with a large air gap dielectric  C drops and Z rises , with L being about 0.5~1uH/m. So signal reflections start to be noticeable as ringing  when length of jumper >5~10%\$\lambda\$.