Do you mean changing with time? or changing with temperature? or what? by saying "not constant across wires".
across wire length. For example first 1 cm can have 70 Ω, then 10 cm 130 Ω, then 10 cm 20 Ω, etc. These are not real digits, just for explanation what I mean when I told you about "not constant impedance". And when you move wires it leads to impedance change, so signal distortions and reflections inside wires will be changed.
In order to avoid it, you're needs to use proper RF connectors and coax cable. With proper soldering or crimping, this is also very important. The coax cable has constant impedance across cable length. And RF connectors are designed in such way to avoid significant impedance change across connection.
Ok, forget about banana, how about this that is sold on ebay just like all "legit" probes
it has the same problem. I have such cables, they works up to 3-7 MHz, above that reflections and distortions are very significant. Also they have high loss at MHz frequency.
You're needs to understand that RF signal is transfered not through wires, but through space between wires. So, the size between wires and insulator material is very important here. Wire geometry also very important. At VHF frequency even a drop of solder can significantly affect impedance.
This is why banana plugs are not applicable for RF signals. You can use it for low frequency signals such as audio below 20 kHz, but if you needs to work with MHz signals, then you're needs coax cables and proper RF connectors.