Make object fall with constant speed!

Well you could either avoid using the physics engine and instead set the transform directly, or you could perhaps set the rigidbodies velocity every frame.

Moving object position using transform:

 function Update() {
 
    transform.position -= transform.up * Time.deltaTime * 5;
 }

Moving object by directly influencing the rigidbodys' velocity every frame:

 function Update() {
 
    rigidbody.velocity = Vector3(0, -5, 0);
 }

rigidbody.velocity