rigidbody constrain y position code example
Example: unity rigidbody constraints
m_Rigidbody = GetComponent<Rigidbody>();
//This locks the RigidBody so that it does not move or rotate in the Z axis.
m_Rigidbody.constraints = RigidbodyConstraints.FreezePositionZ | RigidbodyConstraints.FreezeRotationZ;