Vector3 coordinate and speed on y-axis

I want to move an object only in y-axis following other object. So I used the code below,

transform.position = new Vector3 (28.52172f, point.y, 13.0f);	

but I also want to set it to move in constant speed. For Vector3 slerp, lerp etc moves to targe.position, but I want only target.position.y and freeze x and z axis (with constant speed)

Your help is highly appreciated.
Thanks for advise.

If you want a smooth follow, you can use Mathf.Lerp instead of Vector3.Lerp. Vector3.Lerp is designed for smoothing an entire Vector3 structure, but Mathf.Lerp will work on only one float. Here, you want to smoothly move the object on one axis, and the axes of a Vector3 are defined by 3 floats. So, instead of lerping the Vector3, you just lerp the y axis. For example:

transform.position = new Vector3 (28.52172f, Mathf.Lerp(transform.position.y, point.y, 2*Time.deltaTime), 13.0f);